This invention relates generally to techniques used in geophysical well logging, and more particularly to new techniques for automatically processing dipmeter signals or displacement measurements obtained between these signals to produce more accurate dip and azimuth representations of subsurface formations.
A common method of measuring the dip angle and direction or azimuth of subsurface formations employs a dipmeter tool passed through a borehole drilled into the subsurface formations. This tool may apply any of numerous means to obtain geophysical signals representative of variations of a particular formation characteristic, such as its resistivity. One such tool is described in the paper: "The High Resolution Dipmeter Tool", by L. A. Allaud and J. Ringot, published in the May-June 1969 issue of The Log Analyst.
Dip and azimuth measurements representing the inclination of a formation characteristic or feature may be determined from dipmeter signals containing information representing the intersection of such a feature at three or more radially spaced points on the borehole surface. The displacement between two points intersecting a common feature may be determined, under favorable circumstances, by correlating pairs of the dipmeter signals, each having a similar response to the common feature. Two displacements between three different points determine the position of a plane. The position of the plane is conveniently expressed by its dip .theta., an angle measured from a reference (usually horizontal) plane and its azimuth .PHI., an angle measured from a reference direction (usually true North). Typically, the dipmeter signals are recorded on computer compatible magnetic tape at the well site for later processing. The recorded signals are processed using any of several techniques. Manual, semi-automatic and fully automatic processing may be used with the automatic processing before performed with either analog or digital computers. When digital computers are used, a computer program is also required.
A computer program to perform the digital processing operations is described in a paper, "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tape" by J. H. Moran, et al and published in the July, 1962 issue of the Journal of Petroleum Technology. An additional computer program is described in the paper, "Computer Methods of Diplog Correlation" by L. G. Schoonover, et al, pages 31-38, published in the February 1973 issue of Society of Petroleum Engineers Journal. Further, programs to process digitally-taped dipmeter data may be obtained from digital computer manufactures, such as IBM.
Results from digital processing are normally presented in tabular listings as dip and azimuth measurements versus borehole depth. When desired, the individual displacements found between the correlated curve pairs which led to the dip and azimuth values may be also presented. Further, most such programs will provide the ability to vary both the length of the correlation interval and the step used to move this interval between each correlation sequence. For the next sequence, the same correlation length is used, but the actual interval correlated is moved by one correlation step length.
At each step or depth level, one sequence of displacements between various pairs of signal combinations may be obtained. A typical sequence includes at least two displacements but may include a round of up to six displacements in each sequence when four separate signals are employed, for example. When a round of more than two displacements in one sequence is obtained, the displacements may be combined into many more possibly different combinations, each combination corresponding to perhaps a different dip and azimuth measurement. Since only two related displacements are required, it is common practice to utilize only what appears to be the two best qualified displacements. All others are discarded without further consideration, thereby producing only one result per sequence. Further, little is retained as to the position of the sources or dipmeter pads corresponding to the utilized displacements.
When large numbers of measurements result, as from recent high resolution dipmeter techniques, tabular listings are usually augmented by graphic presentations of dip and azimuth representations. The graphic displays vary with the interpretation objective, depending upon whether the purpose is for stratigraphic or structural studies. Accordingly, relationships between the corresponding dip and azimuth measurements and their continuity with depth are considered in different manners.
For stratigraphic purposes, trends of adjacent dip measurements with depth are usually used to classify the measurements. For example, measurements representing a trend of rapidly increasing dip with depth will be considered separately from measurements representing a trend of rapidly decreasing dip with depth.
In the stratigraphic analysis, it is important that the azimuth of these dips must remain substantially constant and thereby represent the general direction of sediment transport or perhaps the probable direction of down dip thickening. Also, dipmeter results are combined in a given analysis from intervals corresponding to a given depositional or stratigraphic unit.
Graphic displays used for stratigraphic analysis often ignore the actual depths once the above dip versus depth trend for a given azimuth range qualifies a group of measurements. Further, since in many cases the actual dip angle is not important and only the dip azimuth is significant, the dip angle may be completely ignored in the graphic display. Such displays are designed to statistically determine the azimuth corresponding to a primary and perhaps a secondary direction of transport or deposition.
Graphic displays used in stratigraphic analysis are typically the azimuth frequency plot (no dip or depth representation) and the Schmidt net and the Stereonet (azimuth versus dip but still no depth representation). These nets and several variations thereof have known statistical characteristics in that they may enhance either low or high dip measurement point groupings. Note that in their use, the dip and azimuth value for each measurement is combined and represented by a point in these nets. A description of some of these displays and their application is given in the paper "Stratigraphic Applications of Dipmeter Data in Mid-Continent" by R. L. Campbell, Jr., published September 1968 in the American Association of Petroleum Geologists Bulletin.
Stratigraphic and structural analyses distinguish themselves in the type of information needed. In stratigraphic analysis, the dipmeter signals hopefully represent bedding planes within the boundaries of a given geological unit. These bedding planes have little, if any, regional extent. In structural analysis, a deliberate attempt may be made to mask out such sedimentary features in favor of enhancing the boundaries of the individual strata.
Short lengths (1 to 2 or 3 feet) of dipmeter signals are correlated to obtain stratigraphic information while long lengths (10 to 20 or 30 feet) of signals are often correlated to obtain structural information. While use of long correlation lengths to obtain structural dip has been standard practice for some time, there are certain disadvantages associated with this practice. One is that the use of long correlation lengths masks dip patterns needed for stratigraphic analysis, thus additional computations must be made using a short length to obtain stratigraphic information. Another is that most long correlation length techniques may be influenced by frequently occurring stratigraphic features having a common dip and direction, even though each such feature is less pronounced then the structural feature. Thus, the use of long correlation lengths does not assure obtaining accurate structural dip information. Yet another disadvantage is that current correlation techniques tend to ignore possibly objectionable effects of rotation of the dipmeter tool within the long correlation interval.
The preferred approach is to obtain the detailed information available only from short correlation intervals and then apply previously mentioned trend analysis to separate the stratigraphic and structural dips. However, as the correlation interval is shortened, the probability of obtaining a completely erroneous displacement increases substantially. The wrong peak on the correlation function produced in the correlation process may be used to determine the displacement. Such invalid displacements may be combined with valid displacements and produce an erroneous dip which add scatter and confuse valid trends or when systematically erroneous, may even appear as false trends.
As a compromise, longer correlation intervals than are actually desired are employed to artificially reduce this scatter to an acceptable level so that any valid trend which may be present might be found.
It is therefore an object of this invention to provide a technique to reduce the scatter in dip and azimuth measurements determined from short correlation intervals.
One technique which is employed to reduce scatter and find dip and azimuth trends is to average long intervals of dip measurements obtained from much shorter intervals. Unfortunately, the valid trends present only as short intervals may be masked completely by such an averaging process. Further, the resolution and position of the correct peak obtained by correlating short intervals tends to vary considerably, consequently, the corresponding displacements lack accuracy. Certain combinations of such displacements may compound the variation and introduce unacceptable inaccuracies in the resulting dip and azimuth measurements.
It is therefore an additional object of the present invention to provide a technique to improve the accuracy and reduce the scatter of dip and azimuth measurements without necessitating long interval averaging.
Some of the averaging techniques include a preliminary process of sorting or discarding apparently stray dips before averaging to prevent their contributing to the average. This process adds both time delays and expense to a process which already produces too few dips for many purposes. Further, some of the apparent strays may actually be part of a valid trend which was unfortunately just sampled infrequently. Both the discarding and averaging processes suppress such valid dips.
It is therefore a further object of the present invention to provide an automatic technique to improve the accuracy of dip and azimuth determinations without reducing the number of valid dips or discarding dips because they do not comply with some long interval trend.
When such averaging techniques are employed, the intervals to be averaged are often chosen arbitrarily such as every 100 feet or the like. Yet such zoning or sample grouping is an important factor in most statistical analysis. In some techniques, independent geological information is examined (usually manually) to select specific zones to be averaged. This latter process requires considerable time as well as accurate coordination of the depths of the geological information and the dipmeter information. This depth coordination may be a problem in deviating holes where the dipmeter information might not correspond to true depths. It would therefore be advantageous to have the determination of zones be made from the dipmeter data itself.
It is therefore a further object of the present invention to provide a technique for automatically zoning dipmeter information by analyzing the dipmeter information itself.
As previously discussed, there are prior art techniques for statistically analyzing either the dip or azimuth information for long interval trends. These methods usually employ polar chart representations to classify the dip and/or azimuth measurements. In these plots, the dip varies with distance from either the center or the edge of the plots and the azimuth varies with the radial distribution from the center of the plot.
However, when one considers the type of errors likely to take place in the correlation processes, particularly in deviated holes, it is desirable that any analysis not separate the dip from the azimuth values for the purposes of the analysis. The analysis should be able to detect any interrelationship between the dip and azimuth for the individual measurements. More particularly, the analysis should respect the fact that erroneous displacements can be concealed when expressed only as the resulting dip and azimuth measurements.
It is therefore a further object of the present invention to provide a technique for analyzing displacements and combinations of displacements rather than computing and analyzing the resulting dip or azimuth measurements. Prior art methods of dip and azimuth analysis largely ignore the direction of the borehole when deviated. Yet this may be an important control on the distribution of the measurements. Due to the type of problems associated at times with the borehole tool operation, the position of the tool and the signal sources relative to the borehole deviation and direction should be considered in the analysis in case they are also a factor in the distribution of the measurements.
Therefore, it is a still further object of the present invention to provide a system for analyzing displacements and combinations of displacements which considers the relative position of the borehole and the tool in determining the most valid displacements and combinations thereof.
In accordance with these and other objects of the present invention, apparatus and methods are provided for automatically processing with a machine displacements obtained between geophysical signals derived from sources spaced at different positions to determine and producing recorded representations of the relative position of the features on said signals. In one form of the invention, displacements are obtained between similar features in overlapping intervals of geophysical signals which are derived from the spaced sources. Displacements between said signals which are found to be possibly corresponding are combined and these combinations classified as a function of the relative position of the sources. These classified combinations are analyzed to determine the position of the dominant class, from which the corresponding relative position of the signals may also be determined.
When applied to geophysical signals derived in a borehole from separate dipmeter pads which are known to be spaced at different positions around the borehole, the position of the dominant class and corresponding relative position of the signals may be used to derive the dip and azimuth of a variation in a characteristic of a subsurface earth formation.
In one feature of the invention the displacements are combined to generate for each combination a function representing a displacement relationship between any three related signals. These relationships are analyzed to determine the most valid combination of displacements. The analysis may be made in conjunction with other information such as the position of the signal sources in regard to the deviation of the borehole. In one analysis, the most valid combinations of displacements are those derived from signal sources which are substantially in contact with the borehole wall.
In highly deviated holes, the above valid displacement combinations may be considered as the only possible corresponding displacements which may be combined and further analyzed to determine the dip and azimuth of a particular zone.
In a still further feature of the invention, displacements are analyzed to determine the beginning and ending sequences of displacements corresponding to substantially stable zones of displacements. Displacements from sequences in such stable zones are then combined as possibly corresponding displacements for one analysis while displacements from sequences outside such zones are combined for another analysis.
Since each corresponding pair of displacements in each sequence may be combined to determine a possible position of a formation feature, each displacement combination is retained for classification. Thus, many redundant possible combinations from each sequence, and from sequences in which the displacements were obtained from overlapping signal intervals, are classified.
In the analysis of these classified combinations, some of these combinations may now be discarded, rather than, as in the prior art, discarding the redundant displacements at each sequence without further consideration.
The retention of substantially all possible corresponding displacements and combinations thereof, even though they may appear to be of poor quality, for use in classification and analysis allows their contributions to accumulate with other combinations. Therefore, accumulations of weak but consistent displacements which are in face the correct ones, are not prematurely discarded. Further, apparently good quality but perhaps erroneous or inconsistent displacements will not be selected to compute the only dip and azimuth representations for the sequence.
The techniques of the present invention recognize that what has been previously regarded as ambiguous or lesser quality information often contains important information. By retaining this information until contributed to the analysis, the invention utilizes information which was previously discarded for sometimes arbitrary reasons or, in some cases, never even computed.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.